But though the geological systems stand thus strongly marked off from each other when viewed as a whole, their boundary lines can often be only approximately drawn, thereby reminding us that the divisions are of man's device, and can have had no place in the plans of Him who needs not to chronicle His working by years and ages, but with whom there is no past and no future. One formation insensibly passes into another just as one nation merges into those around it. There are sometimes gaps, however, between the formations, serving to mark out strongly the limits of each,[59] precisely as intervening seas and mountain-chains serve to mark put the boundaries of different peoples and tribes.
[59] Such cases, however, are probably merely local, and may have originated from some features in the ancient physical geography of the districts where they occur. For instance, it has always been thought that palæozoic ages were marked off by a strong line of demarcation from succeeding secondary times. But the gap which occurs in England, France, and Germany, is being slowly filled up from the evidence furnished by other countries, and we shall probably find in the end that the Permian dovetailed with the Trias as closely as the Silurian with the Old Red, or the Lias with the Oolite. In truth, the longer we study the past history of our planet the less do we see of hiatus and chasm and sharp clearly defined boundary line; while the doctrine of a uniform system of laws and arrangements in the physical world, first philosophically propounded in the immortal "Principles" of Sir Charles Lyell, is ever receiving fresh confirmation.
The mineral substances of which these formations consist are comparatively few in number, being chiefly varieties of sandstone, shale, conglomerate, and limestone. One sandstone can often be scarcely distinguished from another, and so also with the other rocks; hence such tests as mineralogical texture supplies can seldom be relied on to determine the age of rocks. We can prove, for example, that a series of limestones in England may be identical in age with a set of sandstones in Sweden, and with a group of shales in America, because they all contain the same or representative genera and species of organic remains. They occupy the same position in the geological scale; that is, the animals whose fossilized remains lie buried in these rocks were all living at the same time, while lime was gathering at the sea-bottom over the site of part of England, and sand was being thrown down upon a portion of what is now Sweden, and mud was accumulating over a submerged area of America. In such cases the differences of mineralogical character go for nothing in determining the age of the rocks; we have to rely solely on the embedded fossils, and on the order of superposition.
Keeping in view, then, that the formations into which the geologist has grouped the stratified portion of the earth's crust have a settled and invariable order of occurrence, that each of them contains its own peculiar and characteristic group of organic remains whereby it can be recognised in any part of the world, and that such remains form often the sole test at once of the geologic age and of the origin of the rocks wherein they lie, we may return to the plan above proposed and endeavour to understand the structure of a coal-field. For this purpose it may be well to select one of the northern coal-fields of Britain, since these perhaps display a greater variety in their organic contents, and bear evidence of more diversified changes in their mode of formation than can be seen in those of the south. The strata that compose the coal-basin of Mid-Lothian will probably best suit our purpose, as they are free from the disturbing effects of those igneous intrusions which play so important a part among similar rocks to the north and west.
The Mid-Lothian coal-field comprises a mass of stratified beds of sandstone, shale, coal, ironstone, and limestone, the united depth of the whole being above 3000 feet. By reference to the annexed Table it will be seen that the lowest beds of the section are chiefly sandstones and shales, extending downwards to an unknown depth, without any coal that can be profitably worked. These under-strata form the Lower Carboniferous group. Above them comes a middle zone in which the characteristic beds are of limestone, comprising the middle portion or Mountain Limestone of the Scottish Carboniferous rocks. The third and highest subdivision forms the Upper Carboniferous group or true Coal Measures, and constitutes the whole of what is properly the Mid-Lothian coal-field. For the sake of noting some of the remarkable changes exhibited in the character of the rocks, it may be well to begin our survey among the upper beds of the under group. Let us take as our base the famous limestone of Burdiehouse, and work our way upward through the four thousand feet of strata that lie piled above it.
VERTICAL SECTION OF THE MID-LOTHIAN COAL-FIELD.
| UPPER CARBONIFEROUS OR COAL-MEASURES. | ||||
 | Flat Coal Group. (Above 1000 feet.) |  | A series of sandstones, shales, and fire-clays, with interbedded seams of coal occupying the central area of Mid-Lothian coal-field. | |
| Roslyn Sandstone Group. (About 1500 feet.) | | ········· ········· ········· | A great series of sandstones and shales with three seams of marine limestone (marked here by dotted lines), With the exception of one or two thin seams it contains no coal, and serves in this way to mark off the coal-bearing beds above from the still richer coal-bearing beds below. | |
| Edge Coal Group. (800-900 feet.) | | A group of sandstones and shales similar to those at the top, and like them abounding in coal seams, some of which are thick and valuable. | ||
| CARBONIFEROUS LIMESTONES. | ||||
| Roman Camp Limestones. (150-200 feet.) | | A set of marine limestones intercalated with sandstones, shales, and a few seams of coal. | |
| LOWER CARBONIFEROUS. | ||||
 | Burdiehouse Limestone. (27 feet.) | | ········· | Sandstones and shales extending to an unknown depth, often with seams of dull-grey compact limestone, rarely of coal. The beds become very red towards the base, and wholly devoid of fossils. |
| Thickness of Lower Carboniferous Rocks unknown, but probably greater than that of the upper. | ||||
The Burdiehouse limestone is twenty-seven feet thick, of a yellowish or bluish-grey colour, very compact, splintery, and often fissile in structure, with a finely striped and laminated appearance, which probably indicates a slow and tranquil origin. It is Crowded with fossils, every fragment when taken up showing its seed-cone, fern-stem, fish-scale, or minute cyprides. All the plants seem to belong to terrestrial species, and have a broken and often a macerated look. Manifestly they never grew where we now find their remains; they must have come drifting down from swamp, or jungle, or hill-side. And so we come to know that during the later ages of the Lower Carboniferous period, there lay somewhere in the neighbourhood of Burdiehouse a land clothed with ferns and club-mosses, and through whose swampy hollows there spread a network of stigmariæ, while sigillariæ waved their fronds high overhead. From what has been said on a previous page we may infer that the climate of the old land was moist and equable like that of New Zealand, nourishing a prolific growth of ferns and other plants comparatively low in the botanical scale. The scenery of the vegetation displayed perhaps no great variety of outline, but exhibited rather an endless succession of the same graceful forms.
But the limestone presents us with other remains than merely those of terrestrial plants. It displays in abundance the minute dissevered cases of cypris, the small crustaceous animal described above. Recent species of this genus inhabit stagnant ponds or the bottoms of gently-flowing rivers, and we hence infer that the ancient species must in like manner have possessed a similar habitat, and consequently that the rocks which preserve their remains must have been deposited in fresh (or, perhaps, brackish) water. Tried by this test the Burdiehouse limestone must be regarded as a lacustrine, or more probably a fluviatile formation, which gathered slowly on an undisturbed bottom swarming with crustaceans and plentifully covered with leaves, branches, rootlets, and other fragments of terrestrial plants brought down by streams from the adjoining land. Thus the inferences drawn from the numerous plants, and from the countless multitude of cypris-cases, come to be mutually corroborative. The former tell us of some neighbouring forest-covered country; the latter lead us, as it were, into its river-mouths, whence we can descry the waving woods on either side.
Still we have not exhausted all the fossil remains of the Burdiehouse rocks. Mingled among the stems of ferns and lepidodendra, and the scattered valves of the cyprides, lie the scales, teeth, and bones, of several large ganoidal fishes, along with entire specimens of the smaller genera. The scales of holoptychius are especially abundant, often crowded together by dozens, and probably not far out of the arrangement they had when grouped on the body of the living animal. Detached teeth of the same fish also frequently occur along with disjointed internal bones. The remains of the contemporary megalichthys likewise abound, more particularly the scales, which have a fine nut-brown colour, and dot the surface of the rock with their bright glittering enamel. Several other smaller ganoids may be met with, especially a small and elegant species of Palæoniscus (P. Robisoni), and one of Eurynotus, a fish remarkable for the great size of its dorsal fin. Not uncommon, too, are the ichthyodoralites of a gigantic placoid—the Gyracanthus formosus—with all their delicately-fretted ornament and a peculiar crystalline glistening surface when broken across, whereby the smallest fragment can be easily distinguished from any other bone in the limestone. Such are the ichthyic remains of the Burdiehouse beds; what deductions can be legitimately drawn from them?